More operational speedup and power saving have been recently requested for electronic devices including a DRAM. As one of means for realizing operational speedup and power saving, it is listed to use SOI (Silicon On Insulator) substrate instead of a silicon substrate (bulk silicon substrate) normally used.
An SOI substrate is a substrate having a structure that a silicon region is present on an insulating region and there are several types of SOI-substrate forming methods such as the bonding method, SIMOX (Separator by IMplanted OXygen) method, and ELTRAN (Epitaxial Layer TRANsfer) method.
However, because a conventional SOI-substrate forming method requires a large cost, there is a problem that an SOI substrate is not suitable for a household electronic device such as a DRAM. Moreover, because it is difficult to form a silicon region (element-forming region) having less defects, there is also a problem that a sufficient reliability cannot be obtained compared to the case of using a bulk silicon substrate.